We have developed a sensitive experiment which allows us to observe the onset of hole formation in thin freely-standing polystyrene (PS) films. In the experiment, a constant small pressure difference is applied and maintained across the freely-standing film held at a fixed elevated temperature. The formation of holes due to the thermal instability of the film is detected as a flow of air through the film. We have studied the onset of hole formation in very thin, freely-standing PS films for which the glass transition temperature ' Tg' is 40°C less than the bulk value of 'T g' (bulk) = 98°C. For these films, we observed hole formation, and therefore substantial chain mobility, at temperatures that are less than the bulk value of 'Tg'. However, hole formation is only observed at temperatures that are substantially higher than the ' Tg' value of the films.